Optical recording medium and method for producing such an optical recording medium

ABSTRACT

The present invention relates to a method for producing an optical recording medium. In the method, on the surface of a recording layer formed by spin-coating a first coating solution containing a photo-isomerizable component, a second coating solution that contains a photo-isomerizable component that can be isomerized by radiation having the same wavelength as radiation used for isomerizing the photo-isomerizable component contained in the recording layer and incapable of dissolving the recording layer is spin-coated to form an intermediate layer. Since the intermediate layer thus formed cannot be dissolved by the first coating solution, a recording layer is further laminated on this layer. Thus, it becomes possible to make the recording layer thicker and also to provide a high-density recording characteristic.

BACKGROUND OF THE INVENTION CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims priority under 35 USC 119 from Japanesepatent Application No. 2003-081289, the disclosure of which isincorporated by reference herein.

[0002] 1. Field of the Invention

[0003] The present invention relates to an optical recording medium anda method for producing the optical recording medium. More particularly,the invention relates to an optical recording medium provided with aplurality of recording layers on which information can be recordedholographically are laminated, and a method for producing the opticalrecording medium by spin- coating.

[0004] 2. Description of the Related Art

[0005] Conventionally, rewritable optical disk recording media, such as12 cm-diameter DVD-RAM capable of high-density, double-sided recordingof 5.2 GByte (Gigabyte) with a phase change system, have already beenwidely used. However, in these optical disk recording media, since dataare recorded in a plane, high-density recording is restricted bydiffraction limit of radiation and has come close to a physicallimitation. In order to achieve a higher recording capacity, athree-dimensional (volume) recording system including a depth directionis required. With respect to the above-mentioned volume-type recordingmedium, a volume holography memory capable of volume-recording hologramlattices has been regarded as a prospective medium.

[0006] Here, in order to achieve a large capacity in the volumeholography memory, “a thicker recording layer” is the most importantfactor. In general, the thicker the hologram, the narrower the incidentangle conditions for allowing diffraction, and only a slight offset fromBragg conditions causes diffraction radiation to disappear. The multipleangle method in the volume holography memory utilizes this angleselectivity. In other words, a plurality of holograms are formed in thesame volume, and by controlling the incident angle of reading radiation,it becomes possible to read a desired hologram without crosstalk. Inthis manner, by increasing the film thickness of the recording layer soas to improve the angle selectivity, the multiplicity can be increasedso that the recording capacity is increased.

[0007] In DVDs and the like, in general, the spin-coating has been usedfor forming a recording layer. In the spin-coating, a coating solutionis dropped onto portions near center on a rotating disk, the coatingsolution, driven by centrifugal force, flows and reaches the peripheryof the disk to form a film, excessive coating solution flies out fromthe periphery of the disk, then the solvent is removed from the film.For example, recording layers and non-recording layers are laminatedalternately by using the spin-coating to form a multi-layer holographicmemory (see Japanese Patent Application Laid-Open (JP-A) No. 11-250496).In this example, a solution A prepared by dissolving polymer containinga photo-reactive component in pyridine is spin-coated to form arecording layer having a thickness of 1 μm, and on this recording layer,a solution B prepared by dissolving polyvinyl alcohol in water isspin-coated to form a non-recording layer having a thickness of 8 μm.

[0008] However, it is very difficult to form a recording layer having athickness of several tens μm or more to be used for the volumeholography through single spin-coating process. Since the holographymemory disclosed in JP-A No. 11-250496 is not used for multiplexrecording, the thickness of the recording layer formed in the methoddisclosed in JP-A No. 11-250496 is as thin as approximately 1 μm.Further, since holograms recorded in the laminated recording medium areformed in the respective recording layers for avoiding crosstalks, thismethod is not suitable for the volume holography.

SUMMARY OF THE INVENTION

[0009] Considering the conventional problems, an object of the presentinvention is to provide a method for producing an optical recordingmedium, which can produce an optical recording medium provided with athick recording layer by employing a spin-coating process. Anotherobject of the invention is to provide an optical recording mediumcapable of forming volume holograms over the entire laminated thick filmand a method for producing an optical recording medium which can producesuch an optical medium.

[0010] A method for producing an optical recording medium of theinvention for achieving the objects can be employed for producing anoptical recording medium in which a plurality of recording layers onwhich information can be recorded holographically are laminated on adisc-shaped substrate with intermediate layers between the recordinglayers. The method is characterized by the steps of: applying a firstcoating solution containing a photo-isomerizable component onto asurface of the horizontally-held substrate or intermediate layer; thecoating solution is driven toward and flied out from the periphery ofthe substrate by a centrifugal force generated by rotating the substratewhile the film is dried, to form the recording layer; applying to thesurface of the recording layer a second coating solution incapable ofdissolving the recording layer and containing a photo-isomerizablecomponent that can be isomerized by radiation having the same wavelengthas radiation used for isomerizing a photo-isomerizable componentcontained in the recording layer; and the second coating solution isdriven toward and flied out from the periphery of the substrate by acentrifugal force generated by rotating the substrate while the film isdried, so that an intermediate layer that cannot be dissolved by thefirst coating solution is formed.

[0011] In the method for producing the optical recording medium of theinvention, on the surface of the recording layer formed by spin-coatingthe first coating solution containing a photo-isomerizable component, asecond coating solution that does not dissolve the recording layer isspin-coated to form an intermediate layer. Since the intermediate layerthus formed is not dissolved by the first coating solution, a recordinglayer is further laminated on this layer. Thus, it is possible to makethe recording layer thicker by using the intermediate layer.

[0012] Moreover, the intermediate layer contains a photo-isomerizablecomponent that can be isomerized by radiation having the same wavelengthas that used for isomerizing a photo-isomerizable component contained inthe recording layer so that holograms can be recorded in both of therecording layers and the intermediate layers sandwiched between therecording layers. The photo-isomerizable component contained in theintermediate layer may be the same as or different from thephoto-isomerizable component contained in the recording layer. Theintermediate layer containing the photo-isomerizable component enablesinformation to be recorded also in the intermediate layer to increasethe recording capacity.

[0013] In the method for producing an optical recording medium, aplurality of recording layers and intermediate layers preferably containphoto-isomerizable components that can be photo-isomerized by radiationhaving the same wavelength. With such a constitution, recording andreproducing processes for information recorded on the respectiverecording layers and intermediate layers can be carried out by using asingle radiation source, thereby making it possible to simplify thestructure of the recording and reproducing systems at low costs.Consequently, it becomes possible to provide a wider window for opticalprecision.

[0014] Moreover, the thickness of the intermediate layer is preferablymade thinner than that of the recording layer, and set to ¼ or less ofthe incident wavelength λ. By making the thickness of the intermediatelayer thinner than the thickness of the recording layer, the recordinglayer is further made thicker. Moreover, with the intermediate layerhaving a thickness of ¼ or less of the incident wavelength λ, theincident radiation can pass through without reflection on the interface.Hence, recording and reproducing processes are not influenced by thereflection.

[0015] Furthermore, the viscosity of the first coating solution ispreferably made higher than that of the second coating solution. Bymaking the viscosity of the first coating solution higher, the recordinglayer can be further made thicker. The thickness of the film in whichthe recording layers and the intermediate layers are laminated ispreferably 10 μm or more.

[0016] Here, the photo-isomerizable component may be a polymer componentcontaining a photo-isomerizable organic atomic group or a polymercomponent in which photo-isomerizable organic molecules are dispersed.With respect to the photo-isomerizable organic atomic group or thephoto-isomerizable organic molecules, azobenzene is particularlypreferable.

[0017] Moreover, the optical recording medium of the invention forachieving the objects is an optical recording medium in which aplurality of recording layers on which information can be recordedholographically are laminated on a disc-shaped substrate. The opticalrecording medium is characterized in that the optical recording mediumcomprises alternate lamination of recording layers each containing aphoto-isomerizable component that records holograms byphoto-isomerization; and intermediate layers each of which contains aphoto-isomerizable component that can be isomerized by radiation havingthe same wavelength as radiation used for isomerizing thephoto-isomerizable component contained in the recording layer and iscomposed of a material that can be dissolved or dispersed in a solventthat does not dissolve the recording layer.

[0018] In the optical recording medium of the invention, theintermediate layer contains a photo-isomerizable component that can beisomerized by radiation having the same wavelength as radiation used forthe photo-isomerizable component contained in the recording layer;therefore, holograms can be recorded in both of the recording layers andthe intermediate layer sandwiched between the recording layers. In otherwords, the intermediate layer functions virtually as a recording layer.For example, the recording layers and the intermediate layers can belaminated alternately by forming the recording layer out of awater-insoluble material and forming the intermediate layer out of amaterial that can be dissolved or dispersed in water.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019]FIG. 1 is a perspective view that shows the external appearance ofan optical recording medium according to the present invention.

[0020]FIG. 2 is a cross-sectional view that shows one example of a layerstructure of an optical recording medium according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0021] The following description will discuss embodiments of a methodfor producing an optical recording medium of the present invention.

[0022] An embodiment of the invention is a method (U) for producing anoptical recording medium comprising a plurality of recording layers on adisc-shaped substrate on which information can be recordedholographically, and intermediate layers provided between the recordinglayers, the method comprising:

[0023] applying a first coating solution containing a photo-isomerizablecomponent, which records information holographically byphoto-isomerization onto a surface of one of the substrate or theintermediate layer, which is being held horizontally;

[0024] allowing the coating solution to flow toward a periphery of thesubstrate by a centrifugal force generated by rotating the substratewhile a film is dried, to provide the recording layer;

[0025] applying a second coating solution containing aphoto-isomerizable component that can be isomerized by radiation havingthe same wavelength as radiation used for isomerizing aphoto-isomerizable component contained in the recording layer andincapable of dissolving the recording layer, to a surface of therecording layer; and

[0026] allowing the second coating solution to flow toward the peripheryof the substrate by a centrifugal force generated by rotating thesubstrate while a film is dried, to provide an intermediate layer thatcannot be dissolved by the first coating solution.

[0027] Another embodiment of the invention is the method (U) forproducing an optical recording medium, wherein each of the recordinglayers contain photo-isomerizable components that can be isomerized byradiation having a same wavelength.

[0028] Another embodiment of the invention is the method (U) forproducing an optical recording medium, wherein a thickness of theintermediate layer is thinner than a thickness of the recording layer.

[0029] Another embodiment of the invention is the method (U) forproducing an optical recording medium, wherein a thickness of theintermediate layer is no more than ¼ of an incident wavelength λ.

[0030] Another embodiment of the invention is the method (U) forproducing an optical recording medium, wherein a viscosity of the firstcoating solution is higher than a viscosity of the second coatingsolution.

[0031] Another embodiment of the invention is the method (U) forproducing an optical recording medium, wherein the photo-isomerizablecomponent is a polymer component containing a photo-isomerizable organicatomic group or a polymer component in which photo-isomerizable organicmolecules are dispersed.

[0032] Another embodiment of the invention is the method (U) forproducing an optical recording medium, wherein the photo-isomerizablecomponent is a polymer component containing a photo-isomerizable organicatomic group or a polymer component in which photo-isomerizable organicmolecules are dispersed, and the photo-isomerizable organic atomic groupor the photo-isomerizable organic molecules are azobenzene.

[0033] Another embodiment of the invention is the method (U) forproducing an optical recording medium, wherein a laminated filmconstituted by the recording layers and the intermediate layers has athickness of at least 10 μm.

[0034] Another embodiment of the invention is an optical recordingmedium (V) comprising a plurality of recording layers on whichinformation can be recorded holographically on a disc-shaped substrate,wherein

[0035] the recording layers each contain photo-isomerizable componentsthat record holograms through photo-isomerization, and

[0036] intermediate layers, each of which contains photo-isomerizablecomponents that can be isomerized by radiation having a same wavelengthas radiation used for isomerizing the photo-isomerizable componentcontained in the recording layer and is composed of a material that canbe one of dissolved or dispersed in a solvent that does not dissolve therecording layer,

[0037] are laminated alternately in the optical recording medium.

[0038] Another embodiment of the invention is the optical recordingmedium (V), wherein the recording layer is composed of a water-insolublematerial and the intermediate layer is composed of a material thatcannot be dissolved in water.

[0039] Another embodiment of the invention is the optical recordingmedium (V), wherein the recording layers contain photo-isomerizablecomponents that can be isomerized by radiation having the samewavelength.

[0040] Another embodiment of the invention is the optical recordingmedium (V), wherein a thickness of the intermediate layer is thinnerthan the thickness of the recording layer.

[0041] Another embodiment of the invention is the optical recordingmedium (V), wherein a thickness of the intermediate layer is no morethan ¼ of an incident wavelength λ.

[0042] Another embodiment of the invention is the optical recordingmedium (V), wherein the photo-isomerizable component is a polymercomponent containing a photo-isomerizable organic atomic group or apolymer component in which photo-isomerizable organic molecules aredispersed.

[0043] Another embodiment of the invention is the optical recordingmedium (V), wherein the photo-isomerizable component is a polymercomponent containing a photo-isomerizable organic atomic group or apolymer component in which photo-isomerizable organic molecules aredispersed, and the photo-isomerizable organic atomic group or thephoto-isomerizable organic molecules are azobenzene.

[0044] Still another embodiment of the invention is the opticalrecording medium (V), wherein a laminated film constituted by therecording layers and the intermediate layers has a thickness of at least10 μm.

Optical recording medium

[0045] First, the structure of an optical recording medium obtained bythe production method of the invention is explained.

[0046] As shown in FIG. 1, an optical recording medium 35 is adisc-shaped recording medium having a center hole 10 formed in thecenter. As shown in FIG. 2, the optical recording medium 35 isconstituted by a plurality of recording layers 14 and a plurality ofintermediate layers 16 that are laminated alternately on a disc-shapedtransparent substrate 12. FIG. 2 shows an example in which fiverecording layers 14 and four intermediate layers 16 are provided,however, the number of the laminated layers is not limited to thisexample.

[0047] With respect to the transparent substrate 12, a substrate, suchas a quartz substrate, a glass substrate and a plastic substrate, can beused. Here, “transparent” refers to the fact that the substrate istransparent with respect to recording radiation and reproducingradiation. With respect to the material for the plastic substrate,examples thereof include polycarbonate; acrylic resins such aspolymethyl methacrylate; vinylchloride-based resins such aspolyvinylchloride and a vinylchloride copolymer; epoxy resins; amorphouspolyolefin and polyester; all of which are made to have a low index ofdouble refraction. From the viewpoint of moisture resistance,dimensional stability and price, polycarbonate is particularlypreferable. The thickness of the transparent substrate 12 is preferablyfrom 0.01 to 5 mm. Here, the transparent substrate 12 may be providedwith a concave and convex pattern (pregrooves) that represents guidegrooves for use in tracking or information such as address signals.

[0048] Holograms can be recorded in the recording layers 14 and theintermediate layers 16 by changing the refractive indexes or absorptioncoefficients of the recording layers 14 and the intermediate layers 16through photo-isomerization. The recording layers 14 or the intermediatelayers 16 may be composed of any material as long as it maintains thechanged refractive indexes or absorption coefficients at normaltemperature. Examples of the preferable material includephoto-responsive materials that exhibit photo-induced double refraction.Those materials that exhibit photo-induced double refraction respond toa polarizing state of incident radiation, and can record the polarizingdirection of the incident radiation. Here, the optical recording mediumin which photo-induced double refraction holograms corresponding to thepolarization distribution can be recorded is referred to as an opticalrecording medium sensitive to polarization.

[0049] As material that exhibits photo-induced double refraction, apolymer or a polymer crystal having a photo-isomerizable group on itsside chain, or a polymer in which photo-isomerizable molecules aredispersed, is preferably used. With respect to the photo-isomerizablegroup or molecules, for example, those materials having an azobenzeneskeleton are preferably used.

[0050] The following description will discuss the principle ofphoto-induced double refraction, using azobenzene as an example. Asshown in the following chemical formulae, azobenzene is allowed toexhibit a trans-cis photo-isomerizing property when irradiated withradiation. Prior to the irradiation of a photorecording layer withradiation, many trans-state azobenzene molecules exist in thephotorecording layer. These molecules are randomly oriented, and areisotropic from a macroscopic viewpoint. When linearly polarizingradiation is irradiated to the photorecording layer in a predetermineddirection indicated by an arrow, a trans1-state molecule having anabsorption axis in the same orientation as the polarizing orientation isselectively photo-isomerized into a cis-state. Molecules which have beenrelaxed to take a trans2-state having an absorption axis orthogonal tothe polarizing orientation, no longer absorb radiation, and are fixed inthe state. Consequently, from a macroscopic viewpoint, the anisotropy ofthe absorption coefficient and refractive index, that is, dichroism anddouble refraction, are induced. In general, these characteristics arereferred to as photo-induced double refraction, photo-induced dichroismor photo-induced anisotropy. By applying circular-polarizing ornon-polarizing radiation, the excited anisotropy can be erased.

[0051] The orientation of such a polymer having a photo-isomerizablegroup is also changed by the photo-isomerization to induce greaterdouble refraction. The double refraction thus induced is stable at atemperature lower than the glass transition temperature of the polymer,and preferably used for hologram recording.

[0052] Preferable examples of the materials constituting the recordinglayer 14 include polyester (hereinafter, referred to as “azopolymer”)having azobenzene in its side chain, which is represented by thefollowing formula (1). Azobenzene on the side chain of this polyester isphoto-isomerized to give photo-induced anisotropy. The intensity andpolarizing direction of signal radiation can be recorded as holograms byusing the polyester due to the photo-induced anisotropy. Amongpolyesters of this kind, in particular, polyesters having cyanobenzeneon their side chain are preferably used. (“Holographic recording andretrieval of polarized radiation by use of polyester containingcyanoazobenzene units in the side chain”, K. Kawano. T. Ishii, J.Minabe, Ti. Niitsu, Y. Nishikata and K. Baba, Opt. Lett. Vol. 24 (1999)pp. 1269-1271).

[0053] In the general formula (1), X represents a cyano group, a methylgroup, a methoxy group or a nitro group, and Y represents a divalentconnecting group that forms an ether bond, a ketone bond or a sulfonebond. Each of 1 and m is an integer of 2 to 18, more preferably, 4 to10, and n is an integer of 5 to 500, more preferably, 10 to 100.

[0054] The film thickness of each recording layer 14 is preferably in arange of 1 to 100 μm, more preferably, 5 to 30 μm. Moreover, the totalof the film thicknesses of the recording layers 14 in the entire mediumis preferably in the range of 10 to 1000 μm, more preferably, 100 to 500μm. The method for providing the above-mentioned recording layer 14 willbe described later.

[0055] In order to increase the total of the film thicknesses of therecording layers in the entire medium and impart a function of recordingvolume holograms to the recording medium by laminating recording layers14, intermediate layers 16 are inserted between the recording layers 14.With respect to the material to be used for forming the intermediatelayer 16, examples thereof include water-soluble materials such aspolyvinyl alcohol (PVA), polyethylene glycol, cationized cellulose,sodium carboxymethylcellulose, water-soluble chitosan, gelatin, amyloseand pectinic acid. Moreover, as described above, a hologram recordingmaterial is added to the intermediate layers 16 as well as recordinglayers 14. Since the hologram recording material is contained in theintermediate layer 16, the medium as a whole is utilized as a recordinglayer.

[0056] The thickness of the intermediate layer 16 is preferably madethinner than that of the recording layer 14. More specifically, thethickness of the intermediate layer 16 is preferably from 0.05 to 0.2μm, more preferably, from ¼ or less of the incident wavelength λ. Bymaking the intermediate layer 16 thinner, the recording layer 14 isvirtually made thicker. By making the intermediate layer 16 thinner to ¼or less of the incident wavelength λ, the reflection on the interface isprevented. The method for providing the above-mentioned intermediatelayer 16 will be described layer.

Method for producing an Optical Recording Medium

[0057] The following description will discuss the method for producingan optical recording medium by which the above-mentioned opticalrecording medium is produced. In the production method of the invention,the following “forming process of a recording layer” and “formingprocess of an intermediate layer” are repeated alternately to produce anoptical recording medium in which recording layers and intermediatelayers are alternately laminated.

[0058] (Forming Process of Recording Layers)

[0059] The recording layer 14 is provided by spin-coating a coatingsolution for the recording layer on the surface of a transparentsubstrate 12 or an intermediate layer 16. The spin-coating process iscarried out by dropping the coating solution on a portion near center ona rotating disc-shaped transparent substrate 1; the coating solution isdriven toward the periphery of the transparent substrate 12 bycentrifugal force to form a film; an excessive coating solution is fliedout from the periphery of the transparent substrate 12; and the solventis removed from the film.

[0060] The coating solution for the recording layer is prepared bydissolving the above-mentioned hologram recording material in anappropriate solvent. With respect to the solvent used for preparing thecoating solution, examples thereof include: tetrahydrofran (THF),chloroform, methylene chloride and the like. These may be used alone, ortwo of more of these may be used in combination. The solvent isappropriately selected by considering the solubility of the materialused in the recording layers 14 and the solubility of the materialconstituting the intermediate layers 16, in order to completely dissolvethe material to be used in the recording layer 14 and also to preventthe intermediate layer 16 from being dissolved during coating.

[0061] The concentration of the coating solution for the recording layeris preferably from 0.01 to 50% by mass, more preferably, from 0.5 to 10%by mass, in order to form a uniform and thick film. The hologramrecording material and the like are evenly dissolved in the solvent byusing a method such as an ultrasonic treatment, a homogenizer treatment,and a heating treatment. Here, various additives, such as anantioxidant, a UV absorbing agent, a plasticizer and a lubricant may befurther added to the coating solution, depending on its objects.

[0062] The amount of the coating solution to be coated is preferablyfrom 100 to 5000 ml/m², more preferably, from 200 to 1000 ml/m². Therevolutions rate of the substrate in a period from the addition of thecoating solution to the completion of the drying process is preferablyfrom 10 to 50000 rpm, more preferably, from 100 to 10000 rpm. Thecoating temperature is preferably from room temperature to 130° C., morepreferably, from 20 to 100° C. Moreover, the relative humidity at thetime of coating is preferably from 5 to 80% RH, more preferably, from 5to 70% RH. Moreover, a drying process may be further added thereto.

[0063] (Forming Process of Intermediate Layers)

[0064] The intermediate layer 16 is provided by spin-coating a coatingsolution for the intermediate layer on the surface of the recordinglayer 14. The coating solution for the intermediate layer is prepared bydissolving the above-mentioned materials that will constitute theintermediate layer in an appropriate solvent. With respect to thesolvent used for preparing the coating solution, a solvent that does notdissolve the materials constituting the recording layer 14 is required.An example of such a solvent is water.

[0065] The concentration of the coating solution for the intermediatelayer is preferably from 0.01 to 50% by mass, more preferably, from 0.1to 10% by mass.

[0066] The amount of the coating solution to be coated is preferablyfrom 100 to 5000 ml/m², more preferably, from 200 to 1000 ml/m². Therevolution rate of the substrate in a period from the addition of thecoating solution to the completion of the drying process is preferablyfrom 10 to 50000 rpm, more preferably, from 100 to 10000 rpm. Thecoating temperature is preferably from room temperature to 130° C., morepreferably, from 20 to 100° C. Moreover, the relative humidity at thetime of coating is preferably from 5 to 80% RH, more preferably, from 5to 70% RH. Moreover, a drying process may be further added thereto.

[0067] As described above, the coating solution that does not dissolvethe recording layer is spin-coated on a surface of the recording layerformed by the spin-coating, to form an intermediate layer. And arecording layer can be further laminated on the surface of theintermediate layer thus formed; therefore, it becomes possible to makethe laminated film thicker by using the intermediate layers.

[0068] In the resulting optical recording medium, the intermediate layeralso contains a hologram recording material that can be isomerized torecord holograms upon application of radiation having the samewavelength as radiation used for isomerizing the hologram recordingmaterial contained in the recording layer; thus, the holograms arerecorded in the recording layers and the intermediate layer sandwichedbetween the recording layers. Here, in order to record volume-typeholograms, a total thickness of the film constituted by the recordinglayers and the intermediate layers is preferably at least 10 μm.

[0069] According to the method for producing the optical recordingmedium of the invention, the coating solution that does not dissolve therecording layer is spin-coated on the surface of the recording layer, toform the intermediate layer. And a recording layer can be furtherlaminated on the surface of the intermediate layer thus formed;therefore, the resulting effect is that the recording layer is madethicker by using the intermediate layers.

[0070] Moreover, according to the optical recording medium of theinvention and the producing method thereof, the intermediate layercontains a photo-isomerizable component that can be isomerized uponapplication of radiation having the same wavelength as radiation usedfor isomerizing the photo-isomerizable component contained in therecording layer. Therefore, it is possible to form volume-type hologramsover the entire thick film in which the layers are laminated.

What is claimed is:
 1. A method for producing an optical recordingmedium comprising a plurality of recording layers on a disc-shapedsubstrate on which information can be recorded holographically, andintermediate layers provided between the recording layers, the methodcomprising: applying a first coating solution containing aphoto-isomerizable component, which records information holographicallyby photo-isomerization onto a surface of one of the substrate or theintermediate layer, which is being held horizontally; allowing thecoating solution to flow toward a periphery of the substrate by acentrifugal force generated by rotating the substrate while a film isdried, to provide the recording layer; applying a second coatingsolution containing a photo-isomerizable component that can beisomerized by radiation having the same wavelength as radiation used forisomerizing a photo-isomerizable component contained in the recordinglayer and incapable of dissolving the recording layer, to a surface ofthe recording layer; and allowing the second coating solution to flowtoward the periphery of the substrate by a centrifugal force generatedby rotating the substrate while a film is dried, to provide anintermediate layer that cannot be dissolved by the first coatingsolution.
 2. A method for producing an optical recording mediumaccording to claim 1, wherein each of the recording layers containphoto-isomerizable components that can be isomerized by radiation havinga same wavelength.
 3. A method for producing an optical recording mediumaccording to claim 1, wherein a thickness of the intermediate layer isthinner than a thickness of the recording layer.
 4. A method forproducing an optical recording medium according to claim 1, wherein athickness of the intermediate layer is no more than ¼ of an incidentwavelength λ.
 5. A method for producing an optical recording mediumaccording to claim 1, wherein a viscosity of the first coating solutionis higher than a viscosity of the second coating solution.
 6. A methodfor producing an optical recording medium according to claim 1, whereinthe photo-isomerizable component is a polymer component containing aphoto-isomerizable organic atomic group or a polymer component in whichphoto-isomerizable organic molecules are dispersed.
 7. A method forproducing an optical recording medium according to claim 6, wherein thephoto-isomerizable organic atomic group or the photo-isomerizableorganic molecules are azobenzene.
 8. A method for producing an opticalrecording medium according to claim 1, wherein a laminated filmconstituted by the recording layers and the intermediate layers has athickness of at least 10 μm.
 9. An optical recording medium comprising aplurality of recording layers on which information can be recordedholographically on a disc-shaped substrate, wherein the recording layerseach contain photo-isomerizable components that record holograms throughphoto-isomerization, and intermediate layers, each of which containsphoto-isomerizable components that can be isomerized by radiation havinga same wavelength as radiation used for isomerizing thephoto-isomerizable component contained in the recording layer and iscomposed of a material that can be one of dissolved or dispersed in asolvent that does not dissolve the recording layer, are laminatedalternately in the optical recording medium.
 10. An optical recordingmedium according to claim 9, wherein the recording layer is composed ofa water-insoluble material and the intermediate layer is composed of amaterial that cannot be dissolved in water.
 11. An optical recordingmedium according to claim 9, wherein the recording layers containphoto-isomerizable components that can be isomerized by radiation havingthe same wavelength.
 12. An optical recording medium according to claim9, wherein a thickness of the intermediate layer is thinner than thethickness of the recording layer.
 13. An optical recording mediumaccording to claim 9, wherein a thickness of the intermediate layer isno more than ¼ of an incident wavelength λ.
 14. An optical recordingmedium according to claim 9, wherein the photo-isomerizable component isa polymer component containing a photo-isomerizable organic atomic groupor a polymer component in which photo-isomerizable organic molecules aredispersed.
 15. An optical recording medium according to claim 14,wherein the photo-isomerizable organic atomic group or thephoto-isomerizable organic molecules are azobenzene.
 16. An opticalrecording medium according to claim 9, wherein a laminated filmconstituted by the recording layers and the intermediate layers has athickness of at least 10 μm.